The <geo:geo> <geo:format>kml</geo:format> </geo:geo> line, if found in the normal sitemap, produces an "Invalid XML tag" error. The only way to fix it is to separate out the geo-sitemap, as it's own sitemap. Having multiple sitemaps is not a problem, but it might be a little more work for some.

The thing is, it was assumed that this was a temporary requirement and that Google would allow you to add geo specific data elements to your normal sitemap. Over a year later, it is still an issue and new threads are asking why.